Continuous conveying, mixing and/or kneading of media is part of the basic operations of process engineering. The material for mixing and/or conveying can be very different substances. The chemical and physical properties of these substances, their state of aggregation and their behavior under pressure and heat can be correspondingly different. The prior art provided in mechanical engineering include a wide variety of mechanisms such as kneaders for chocolate composition, the production of semifinished plastics by extrusion, the processing of flowable plastics granules with a glass fiber content for injection molding, and the disposal of biologically active waste.
For all these tasks there are known machines which have, depending on the requirements, cooled or heated housings and at least two rotating, mutually meshing conveying screws. On such type of mechanism is called a retruder. Such a device is disclosed and claimed in application Ser. No. 08/211,008, by the invertor, filed on Jul. 18, 1994. This differs from an extruder, which is an injection type of device and produces a high pressure at the end of the worm. A retruder is not an injection device and its object is to reduce pressure at the worm outlet. Further, while the object of an extruder is to generate pressure, a retruder produces torsional forces. Also, the outlet of an extruder is in the direction of the worms whereas in a retruder it is generally perpendicular.
The retruder produces a pressure build-up by forcibly conveying the medium in the individual, closed windings of the screw channel, without having a reactive effect on the mass transport. One of the weak points of this construction is the housing, since the conveying pressure produces peripheral stresses in the housing casing and the friction of the medium results in abrasion and wear of the housing inner face. In particular, this disadvantage becomes important when processing abrasive media or when the high pressures in the medium for the purpose of increasing the frictional forces in the medium are themselves advantageous. This is the case with kneading tasks if the efficiency of kneading is increased or for example in the case of a frictional reactor which is to be operated at high operating pressures as a thermo-mechanical process unit for a biomass. According to the prior art, either the entire housing of the screws, or an integrated plating of the housing, or the surface of the screws, is produced from wear-resistant materials in order to locally protect the locations at risk from material abrasion Cobalt and nickel alloys according to DIN 8555 and hard metal alloys of known type are standard as a protection against wear.
The disadvantage of this solution is the relatively short service life of the protection against wear at the exposed locations. This is because it is precisely at the line of penetration of the screw casing where the mechanical abrasion is the greatest that the plating has to be applied and the localized reduction in the protection against wear has to be accepted.
A further disadvantage of the twin screw retruders is the fact that there is no means of optimizing the screw extrusion in operation other than the relatively complicated speed regulation of the screws.